1. Field of the Invention
The present invention relates to a curable silicone composition that cures by an addition curing reaction, and relates particularly to an addition-curable silicone composition that produces a cured product which has favorable rubber-like properties and strength properties, a high degree of transparency and a high refractive index, as well as an optical element encapsulating material formed from such a composition.
2. Description of the Prior Art
An addition-curable silicone composition comprises an organopolysiloxane containing aliphatic unsaturated groups such as alkenyl groups, and cures via a hydrosilylation reaction to generate a cured product. Cured products obtained in this manner exhibit excellent heat resistance, cold resistance and electrical insulation properties, and are also transparent, and are consequently used in a wide variety of optical applications.
Silicone rubbers used in optical applications require a high degree of transparency and a high refractive index. In order to satisfy these requirements, a dimethylsiloxane-diphenylsiloxane copolymer or a methylphenylpolysiloxane is typically used as the raw material for the main backbone within the cured product.
However, synthesis of a polysiloxane of this type that is capable of producing a cured product with a refractive index of 1.54 or greater has proven difficult. Further, branched polysiloxanes containing introduced phenyl groups are capable of producing cured products with a refractive index of approximately 1.53 to 1.54, but these cured products tend to exist in a hard resin-like state that lacks elasticity. Accordingly, compositions comprising a combination of a branched organopolysiloxane and a linear organopolysiloxane have been proposed, but these are not entirely satisfactory in terms of their transparency, refractive index and elasticity and the like (see Patent Documents 1 to 7).    [Patent Document 1] JP 2005-307015 A    [Patent Document 2] US 2006/0073347 A1    [Patent Document 3] US 2004/0116640 A1    [Patent Document 4] US 2004/0178509 A1    [Patent Document 5] US 2006/0081864 A1    [Patent Document 6] JP 2005-076003 A    [Patent Document 7] US 2007/0112147 A1